Cobalt-containing alloys

Abstract

 A wear-resistant alloy, which excluding impurities, has the following composition:

(a) about 50 to 70% of cobalt, nickel and iron; (b) 27 to 35% of chromium;

(c) 5 to 15% of molybdenum and/or tungsten; (d) up to 2.25% of carbon and/or boron; (e) 0 to 3% of silicon and/or manganese; (f) 0 to 5% of one or more of titanium, hafnium, zirconium, vanadium, niobium, and tantalum; (g) 0 to 5% of copper, and (h) 0 to 2% of one or more rare earths. The cobalt is the range 25 to 40%, and the nickel in the range 4 to 12%. There is from Oto7.5%of constituents (f), (g) and (h). The iron is present in a quantity not exceeding 25%. If there is 2% or more of carbon and/or boron, there is more than 30% chromium. (All percentages by weight).

A welding or surfacing consumable which (ignoring the effect of dilution by substrate) is capable of depositing such an alloy may also be made.

Description

[0001] This invention relates to alloys which contain cobalt and which are resistant to wear and to surfacing and welding consumables which are capable of depositing such alloys.

[0002] Wear-resistant alloys based on the Co-Cr-W-C quaternary system are well known. They comprise a dispersion of hard carbide particles within a strong corrosion-resistant solid solution, rich in cobalt. The strength of the solid solution is such that only moderate volumes of carbide need be employed to achieve a given bulk hardness at both room and elevated temperature. Material toughness is thus appreciable.

[0003] The balance between solid solution strength and carbide level and thus between hardness and toughness of these alloys is such that they are able to withstand wear under both moderate and extreme conditions of temperature and stress (static and cyclic).

[0004] A range of such alloys is available, varying in room temperature hardness from approximately 300 VPN to 700 VPN, to suit different applications.

[0005] Typically, alloys of hardness of approximately 400 VPN contain 1 wt % carbon, 26 wt % chromium and 5 wt % tunqsten, whilst those of hardness approximately 600 VPN contain 2.5 wt % carbon, 33 wt % chromium and 13 wt % tungsten. Not only do the chromium and tungsten contribute to the strength of the solid solution, they also act as carbide formers. The high strength of the cobalt-rich solid solution is associated with a hiah stacking fault density therein.

[0006] In recent years, cobalt has become an expensive commodity. The commercial need to reduce the level of cobalt in such alloys, whilst retaining their excellent wear characteristics, is therefore great.

[0007] Merely to substitute iron and nickel for some of the cobalt is an unsatisfactory expedient. A reduction in cobalt level in these alloys and a corresponding increase in iron or nickel bring about a reduction in solid solution strength and bulk hardness.

[0008] To compensate, increased levels of carbon (and hence carbide) can be employed. Many reduced cobalt alloys contain increased levels of carbide. The toughness of such materials and their ability to withstand severe wear situations are, however, limited. One example of such an alloy is a composition containing 37% by weight of cobalt and nickel; 10% by weight of molybdenum and tungsten; 23% by weight of iron; 27% by weight of chromium and 2.7% by weight of carbon.

[0009] There is a large number of published patent specifications that relate to alloys containing cobalt in smaller proportions than in conventional cobalt-based hard metal alloys. Examples of published patent specifications relating to steels containing nickel and cobalt are Austrian patent specification 136 854, German patent specification 160'410, and UK patent specifications 1 358 284 and 1 381 170. Such alloys, because they are steel, inherently lack the hardness of hard metal alloys based on cobalt. Examples of published patent specifications relating to nickel- based alloys (or alloys containing more nickel than cobalt and iron) which also contain cobalt and iron and US patent specifications 3 692 501 and 3 700 427, UK patent specifications 1 245 158, 880 805, 1 336 409 and 803 253, and Japanese specification 53-147615. UK patent specifications 424 463 and 1 064 109 both describe a range of compositions for hard metal alloys which allows for the cobalt level to fall well below that in conventional cobalt-based hard metal alloys and which also includes nickel and iron. However, the preferred alloys in both specifications contain over 45% by weight of cobalt. The proportions of iron and nickel that are disclosed are suitable more for an alloy containing 45% or more by weight of cobalt than for one containing less than 45% by weight of cobalt. UK patent specification 579 479 relates to a ferrous-based alloy for hard facing purposes containing 10 to 30% cobalt and 13 to 17% nickel. Japanese patent specification 53-14411 relates to a heat resistant alloy containing from 20 to 40% by weight of cobalt, from 10 to 30% by weight of nickel, and some iron. However, it contains less than 4% of molybdenum and tungsten.

[0010] It is an aim of the present invention to provide a range of reduced cobalt alloys which are acceptable alternatives to the Co-Cr-W-C alloys. Compensation is achieved through enhanced strengtheninq of the solid solution, thus retaining to a large extent the balance between solid solution strength and carbide level of the Co-Cr-W-C alloys.

[0011] The invention provides an alloy having (excluding impurities) essentially the following composition, or a surfacing or welding consumable whose formulation is such that on being melted it is capable of producing an alloy which (ignoring the effect of any dilution thereof by substrate material) has essentially the following composition (excluding impurities):



wherein:

(i) cobalt is present in the range 25 to 40% by weight;

(ii) nickel is present in the range 4 to 12% by weight;

(iii) if any is present, the total weight of constituents (f) to (h) is 7.5% or less;

(iv) iron does not exceed 25% by weight;

(v) if there is 2.0% or more by weight of carbon and/or boron present, there is more than 30% by weight of chromium.

[0012] We believe that alloys having properties akin to conventional Co-Cr-W-C alloys can be produced in accordance with this general formulation. The selection of a particular alloy in accordance with the general formulation may be made so as to meet specific reauirements (such as a specified hardness).

[0013] The enhanced stengthening of the solid solution is achieved by using, at relatively high atomic levels, chromium and molybdenum, the latter being in some instances partially or entirely 'replaced' by tungsten.

[0014] Typically, the alloys according to the invention may contain in the order of 30% (by weight) less cobalt than the conventional Co-Cr-W-C alloys with comparable properties.

[0015] We typically prefer to employ more than 10% by weight of iron and typically from 15 to 20% by weight of iron in the composition. The nickel helps to stabilise the face-centred cubic structure of the solid solution. If the alloy is formed into an arc welding consumable, it should be borne in mind that metal from the work being welded may intermix with the weld metal and thus dilute it. Thus, if the arc welding consumable is to be used for the arc weldina of ferrous metal it will typically be desirable for more than 5% by weight of nickel to be present in the consumable.

[0016] If desired, the composition according to the invention may contain no tungsten. Preferably, it contains at least 2% and more preferably at least 5% by weight of molybdenum. Not only does this element provide solid solution strengthening; like chromium, it enhances the resistance to corrosion.

[0017] Preferably, the composition according to the invention contain no boron. If boron is present it preferably does not constitute more than 1% to the total weight of the composition, with carbon also being present.

[0018] If the combined weight of carbon and boron is greater than 1.5% by weight then preferably at least 1.25% of carbon is present. Generally, there is at least 0.3% by weight of carbon present in an alloy according to the invention. Typically, to be suitable for exposure to a wide range of conditions conducive to wear, the composition contains between 0.9 and 1.5% by weight of carbon with there being no boron present. At hiqher carbon levels the alloys tend to have increased hardness but reduced toughness, and vice-versa at lower carbon levels. It is a characteristic feature of alloys according to the invention that they have relatively good wear resistance while containing a relatively low level of cobalt and no exceptionally high level of carbon and boron.

[0019] Typically, from 0.5 to 2.0% (preferably from 0.5 to 1.5%) by weight of silicon and/or manganese are present. These constituents increase the fluidity of the alloys when molten.

[0020] If desired, alloys according to the invention may include up to 5% by weight of one or more elements which are highly active carbide formers. These elements modify the type and morphology of the carbides and release more molybdenum (and/or tungsten) to the solid solution. The highly active carbide former(s) may be selected from tantalum, niobium, hafnium, vandium, titanium and zirconium. There may be up to 5% of such carbide former present. Preferably if one or more of such carbide formers are employed, titanium and/or niobium are selected. It is not essential, however, to include such 'highly active carbide formers'.

[0021] . If desired, alloys according to the invention may contain up to 5% of copper which enhances their resistance to certain corrosive media. Copper is, however, neither an essential nor a preferred constituent of alloys according to the invention.

[0022] If desired, alloys according to the invention may contain up to 2% by weight of one or more rare earths. We believe that the inclusion of such rare earth(s) may be desirable if any alloy according to the invention is required to have particularly good resistance to oxidation at high . temperatures (particularly above 1000°C).

[0023] The preferred rare earth for this purpose is yttrium. However, other rare earths such as lanthanum or cerium may be used instead. The inclusion of a rare earth is not an essential feature of the invention, but is optional.

[0024] A preferred composition is:



wherein hafnium, zirconium, niobium, titanium, tantalum, vanadium, copper and the rare earths does not exceed 7.5% by weight (and preferably does not exceed 5% by weight), and there is not more than 25% by weight of iron present.

[0025] If tungsten is present and the aforesaid preferential carbide formers absent the sum of the percentage weight of molybdenum plus half the percentage weight of tungsten is preferably in the range 5 to 10%.

[0026] Preferably for a general purpose wear resistant alloy the cobalt is present in the range 28 to 38% by weight (and more preferably 28 to 35% by weight).

[0027] Particularly preferred compositions (excluding impurities) are as follows:

[0028] Preferably tungsten and manganese are absent. Typically, there is from 8 to 24% by weight of iron. For a versatile wear-resistant composition, there is typically from 15 to 20% by weight of iron. There is typically from 0.9 to 1.5% by weight of carbon. Typically, there is from 0.5 to 2% (preferably 0.5 to 1.5%) by weight of silicon.

[0029] _' There is, as aforementioned, a range of conventional cobalt- tungsten-chromium-carbon alloys that is commercially available. In this range of alloys the proportions of chromium and tungsten present increase with increasing carbon. Analogously, in the alloys according to the invention, the more carbon that is employed, the greater is the proportion of chromium that should preferably be present, and the greater is the proportion of molybdenum and/or tungsten that should be employed. Moreover, the chromium can typically be at least 31% if the carbon is at least 1.2% by weight.

[0030] Typically, with increasing carbon, the combined proportions of iron, nickel and cobalt in the alloys are reduced. Typically, there is a greater proportion of iron than nickel in an alloy according to the invention. At a carbon and/or boron level of 2.25 % by weight, the combined level of cobalt, nickel and iron may be 50% or just below (down, say, to 48% by weight).

[0031] The alloys according to the present invention may be prepared by mixing their respective ingredients and melting the resultant mixture, typically in a furnace, and typically at a temperature in the order of 1550°C. If desired, the melting may take place in a protective atmosphere of inert gas such as argon or nitrogen or under vacuum. The molten alloy may be formed into a powder by being atomised or by other means, may be made as a casting by, for example, poured into an appropriately shaped mould or may be formed into a cored wire or rod. The powder, wire or rod may be used as hard facing or welding consumables which may, if desired, be coated with a suitable flux. It may also, we believe, be possible to make forgings from alloys according to the invention, if their carbon content is typically less than 1% by weight.

[0032] If desired, engineering or other components may be made as a casting or forging from an alloy according to the invention. Alternatively, such components may be formed by compacting and/or sintering a powder which is an alloy according to the invention.

[0033] The alloys according to the invention may be employed in substantially all applications for which conventional cobalt-based wear resistant alloys are currently used.

[0034] A surfacing consumable according to the invention may be used to deposit a wear resistant coating on a substrate. A welding consumable according to the invention may be used to deposit wear resistant weld metal.

[0035] An alloy according to the invention may be shaped so as to provide - the surfacing or welding consumable. It may also include materials which in use of the consumable are not intended to become an integral part of the metallic deposit. Such materials include for example flux and arc stabilisers. For example a rod or alloy according to the invention may be coated with flux and/or arc stabilisers to form a welding or surfacing consumable according to the invention.

[0036] It is possible to produce what is in effect a chemical equivalent to a welding or surfacing consumable according to the invention. In such an equivalent consumable the alloy is in effect formed in situ as a metallic deposit from the consumable. This metallic deposit may be diluted almost instantaneously by the base or substrate material diffusing into or intermixing with the deposit, or vice versa. For example, some but not all constitutes of the metallic deposit may be present in one discrete part of the consumable and others in another part. Thus, for example, say cobalt and iron are present in an alloy form as a hollow tube, and, say, nickel, molybdenum, chromium, silicon and carbon as a powder within the tube. The tube may also typically contain a flux and arc stabilisinq materials. A typical example is that the tube is of an alloy containing 60% by weight of cobalt and 40% by weight of iron.

[0037] Typically the kind of consumable in which some but not all constituents are present in one discrete part of the consumable and others in another part may take the form of a cored wire. Typically, in such a consumable there will be near to 100% recovery of all the constituents of the consumable in the deposit excluding arc stabilisers, flux and the like, and also with the exception that not all the silicon and/or manganese will in general be recovered, particularly if the consumable contains a flux, in which instance, in use, a part of the silicon and/or manganese tends to enter the slaq which forms on the surface of the metallic deposit. Typically, the consumables may therefore contain 1 to 2% by weight more silicon and/or manganese than it is intended to recover in the deposit.

[0038] The term 'impurities' as used herein encompasses both adventitious impurities and any element or substance deliberately added which does not influence significantly the properties of an alloy accordinq to the invention. For example, small amounts of aluminium (say 1 or 2% by weiqht) may be included without significantly altering the properties of the alloy.

[0039] Desirably, relatively pure constituents are employed in making an alloy acording to the invention as they are when making conventional cobalt-based wear-resistant alloys.

[0040] Examples of alloys according to the invention are set out in Tables 1, 2, 3 and 4. In Table 3, some conventional alloys are shown. Some of their properties are set out in Table 5. Table 6 sets out the composition of conventional cobalt-based alloys. Table 7 illustrates the properties of the alloys set out in Table 6.

[0041] All the alloys that were subjected to the tests set out in Table 5 were formed as identical castings.

[0042] The alloy A (see Table 3) is sold widely under the trade mark 'Stellite 6'. ('Stellite' is a Registered Trade Mark). We believe that alloy No. 3 according to the invention is an acceptable alternative for most, if not all, commercial applications of 'Stellite 6'.

[0043] We have found that at 700°C, the hardness of an alloy such as No. 31 is about 80% of that of alloy A, and at 800°C the UTS of alloy No. 31 is greater than that of alloy A. Moreover, the oxidation resistance of alloy No. 31 in the temperature range 600 to 900°C is approximately equal to that of alloy A. Generally, the resistance of alloy No. 31 to commonly encountered acid solutions is greater than that of alloy A.

[0044] The alloys according to the invention are less affected by iron dilution during arc welding than are conventional cobalt-based hard metal alloys (such as 'Stellite 6'). Selection of an appropriate alloy according to the invention will depend upon the surfacing technique and conditions of deposition.

Claims

1. An alloy or a surfacinq or welding consumable whose formulation is such that on being melted it is capable of producing an alloy which (ignoring the effect of any dilution thereof by substrate material) has essentially the following composition (excluding impurities):

wherein:

(i) cobalt is present in the range 25 to 40% by weight;

(ii) nickel is present in the range 4 to 12% by weight;

(iii) if any is present, the total weight of constituents (f) to (h) is 7.5% or less;

(iv) iron does not exceed 25% by weight;

(v) if there is 2.0% or more by weight of carbon and/or boron, there is more than 30% by weight of chromium.

2. An alloy or consumable as claimed in claim 1, characterised in that the composition includes at least 2% by weight of molybdenum.

3. An alloy or consumable as claimed in claim 1 or claim 2, characterised in that the said composition contains from 5 to 10% by weight of nickel.

4. An alloy or consumable as claimed in any one of the preceding claims, characterised in that there is more iron than nickel in the composition.

5. An alloy or consumable as claimed in any one of the preceding claims, characterised in that the total amount of constituents (f) to (h) is less than 50%.

6. An alloy or consumable as claimed in any one of the preceding claims, characterised in that the aforesaid composition is (excluding impurities) as follows:

7. An alloy or consumable as claimed in claim 6, characterised in that the composition includes 15 to 20% by weight of iron.

8. An alloy or consumable as claimed in any one of claims 1 to 5, characterised in that the composition includes from 27 to 35% by weight of cobalt.

9. An alloy or consumable as claimed in any one of the preceding claims, in which the composition includes from 0.9 to 1.5% by weight of carbon.

10. An alloy or consumable as claimed in any one of the preceding claims in which the composition includes from 0.5 to 1.5% by weight of one or both of silicon and manganese.